Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 10 : Earth History

Ty Robinson

Questions of the Day• How do isotopes help us study the climate history of

Earth?

• What is the ‘Faint Young Sun Paradox’?

• How and why have levels of CO2, CH4 (methane) and O2 changed through Earth’s history?

The Geological Time Scale• Phanerozoic, 540Mya to present day

– “visible life” fossils of plants and animals

• Proterozoic, 2.5Gya to 540Mya– “earlier life” fossils of single-celled organisms

• Archean, 3.9 to 2.5Gya• Hadean, 4.6 to 3.9 Gya

A Word About Isotopes…• isotopes can indicate climate variations on Earth over its

history• need stable isotopes of the same element• the isotopes’ mass difference can lead to “fractionation” of

the isotopes in natural systems at different temperatures• to use as climate indicators we need

– stable isotopes of the same element whose fractionation is temperature dependent

– storage of the fractionated mixture over time– ability to measure the fractionation in samples– a means to date the stored isotope data

• typically use isotopes of carbon, oxygen, hydrogen, and shells or minerals as the storage

Oxygen as a Climate Probe• water has three stable isotopes, H2

16O, H217O and H2

18O • H2

16O is preferentially evaporated from the oceans to form clouds

• during ice ages, the clouds precipitate snow to form ice sheets

• there is a net loss of H216O from the oceans and an

enhancement of H218O vs H2

16O in the oceans

• enrichment in 18O In tiny shelled sea creatures indicates cooler climate conditions

Questions?Why must we use stable isotopes as climate indicators?

radioactive isotopes are dangerous to work with

radioactive isotopes decay, skewing results

stable isotopes are more abundant

stable isotopes are environmentally friendly

Questions?Why is H2

16O preferentially evaporated from the oceans?

it is the lightest of all water isotopes

it is the most abundant of all water isotopes

it is the heaviest of all water isotopes

it is the only water isotope present in ocean waters

The Hadean Eon: “Cryptic” Era• The Earth forms 4.6 Gya• 50-70 My later, a Mars-sized object (0.1Me)

collides with the Earth and the Moon is formed.

The “Hadean”• first 0.6Gy of Earth’s history• Sun was 30% fainter than at present • formed with no “primary” atmosphere, but

outgassed first atmosphere– atmosphere was likely CO2 and water vapor dominated,

no O2 at that time

• zircons show oceans and continental material had formed by 4.4Gya (0.1Gy after formation)

A Planet Under Siege• impacts occurred from formation through until the Late

Heavy Bombardment (about 3.9Gya)• sterilizing impacts probably occurred 6-12 times during

the Hadean– 250-400 km diameter asteroid will vaporize oceans

The Archean Eon • 4.0-2.5 Gya (starts with the oldest

whole rock samples and ends with the rise of oxygen)

• sedimentary rocks are seen after 3.85 Gya

• life first detected by 3.7-3.5 Gya • Earth’s heat flow would have been

much higher and may have resulted in small plates with hotspots in between – produced basaltic proto-continents– Archean continents 10-60% of present

Questions?Why was the Earth’s heat flow higher in the Archean?

leftover heat from the Moon-forming impact

the Sun was brighter in the Archean

plate tectonics were not functioning in the Archean

more the rate of energy production from radioactive decay was higher in the Archean

• 7% of the modern continents contain surviving Archean rocks

The Faint Young Sun Paradox• 3.8 Gya the Sun’s luminosity was 75% of the

present value• an atmosphere with the modern composition,

gives a surface T of 255K (-18C)• yet, water was liquid on the surface• to warm the early Archean above freezing

requires 1000 times the amount of CO2 in the present atmosphere (it would have been 25% of atmosphere)

• evidence from the late Archean indicate that the atmosphere could have been no more than 3% carbon dioxide

…and to make matters worse

• 18O isotope data implies that the Archean ocean may have been 50C, almost twice as warm as today’s hottest oceans!

Archean Atmospheric Composition• ratios of different sulfur isotopes in rocks (combined with other data from

the rock record) suggest that O2 was very low in the Archean atmosphere• O2 rose dramatically in the late Archean/early Proterozoic

The Proterozoic• 2.5 Gya to 540 Mya

• evolution from “small plates” to more modern large plate tectonics

• “supercontinents” may have formed

• near the beginning of the Proterozoic (~2.3Gya) atmospheric O2 rose dramatically from almost nothing in the Archean, to about 10% of the present concentration

The Phanerozoic• 540 Mya to present• characterized by complex

multicellular organisms in the fossil record

• total continental mass comparable to today

• modern-style plate tectonics operating

• O2 is at near modern levels, and is maintained at levels between 15-35% of the atmosphere

Carbon as a Climate Probe

• Carbon has two stable isotopes, 12C and 13C • plants preferentially take up 12C• consequently they preferentially enrich the

atmosphere in 13C• atmospheric carbon enters the oceans through

the chemical erosion of rocks• this fractionation in carbon in the atmosphere is

recorded in the shells of tiny sea creatures• the higher the 13C/12C ratio, the warmer the

climate was at the time the sea creature lived

CO2 in the Phanerozoic

The Earth’s Ice Ages• “Ice Age” is a loose term for times when glaciers

cover down to mid-latitudes.• Ice ages can be triggered by moving continents

poleward, and may have been mediated by life-induced changes in atmospheric composition. – Loss of methane at the Archean/Proterozoic boundary

– Loss of CO2 after widespread colonization of

continents by plants • Snowball Earth?

The Rise of Oxygen

ModernN2 O2 CO2 CH4

N2 CO2 CH4

Questions?Why might you expect that rises in oxygen levels correspond to decreases in carbon dioxide levels?

I wouldn’t expect this!

CO2 can come from burning organic material, which requires O2

oxygenic photosynthesis converts CO2 into O2

oxygen comes from plants while CO2 comes from volcanoes

The Earth’s Ice Ages

Questions of the Day

• How do isotopes help us study the climate history of Earth?

• What is the ‘Faint Young Sun Paradox’?

• How and why have levels of CO2, CH4 (methane) and O2 changed through Earth’s history?

The Phanerozoic (cont). • The Phanerozoic eon is divided into eras,

and eras into periods.

• Boundaries between most of the periods are extinction episodes. – 5 mass extinctions– Possibly due to drastic climate change, ice

ages, large-scale volcanism and impacts

Summary• Earth has evolved from:

– small amounts of continental mass to large continents– small plates to large plates

– low O2 in the Archean to ~20% O2 in the Phanerozoic

• rapid rise at the Archean/Proterozoic boundary (2.5Gya)

– higher CO2 to lower CO2

– possible higher T in the early Archean to lower temperatures

• During its history, Earth has suffered several ice ages, impacts, and volcanic episodes.